Radiation shield for investment molds

ABSTRACT

In making columnar grained or single crystal castings, the mold is heated prior to pouring and a meltable metallic shield is placed on the chill plate to serve as a heat shield between the mold and the plate during heating of the mold and also to prevent condensation of impurities on the chill plate.

United States Patent Barrow et al.

[45] Dec. 19, 1972 [22] Filed:

[54] RADIATION SHIELD FOR INVESTMENT MOLDS [72] Inventors: Robert B. Barrow; Bruce E. Terkelsen, both of Cheshire, Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

Sept. 20, 1971 [21] App1.No.: 182,108

Related U.S. Application Data [62] Division of Ser. No. 31,637, April 24, 1970, Pat. No.

[52] U.S. Cl ..l64/357, 164/361 [51] Int. Cl. ..B22d 15/00 [58] Field of Search ..164/60, 121, 351, 352, 357, 164/361; 249/206 [56] References Cited UNITED STATES PATENTS 3,385,346 5/1968 Fleck ..l64/l2l 2,810,170 10/1957 Pearce .....249/206 X 2,907,083 6/1959 Shakely ..249/206 X Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Att0rneyCharles A. Warren [57] ABSTRACT In making columnar grained or single crystal castings, the mold is heated prior to pouring and a meltable metallic shield is placed on the chill plate to serve as a heat shield between the mold and the plate during heating of the mold and also to prevent condensation of impurities on the chill plate.

I 3 Claims, 3 Drawing Figures QQLQ QQQ U is. QB

RADIATION SHIELD FOR INVESTMENT MOLDS This is a division of Ser. No. 31,637 filed Apr. 24', 1970, for Radiation Shield for Investment Molds, now U.S. Pat. No. 3,633,648.

BACKGROUND OF THE INVENTION As taught in the VerSnyder patent, U.S. Pat. No. 3,260,505 for columnar grained castings, or in the Piearcey'patent, U.S. Pat. No. 3,494,709 for single crystal castings, such castings are best produced in a shell mold, the open bottom end of which rests on a chill plate. The mold is heated to above the melting temperature of the alloy before being poured and a suitable thermal gradient is established in the mold after pouring by the effect of the chill plate. The presence of the chill plate during heating of the mold necessitates a longer period of heating, and during this time impurities that may be within the furnace or within the mold structure including the ceramics or binder may be vaporized. If some of the vaporized impurities enter the mold cavity and are condensed on the chill plate, this condensed material may, when the alloy is poured result in imperfections or porosity in the cast part. Any substantial amount of deposited material on the chill plate may also result in a violent reaction with the alloy further producing imperfections or impurities within the casting.

SUMMARY OF THE INVENTION A feature of this invention is a heat shield positioned in the open end of the mold and resting on or adjacent to the chill plate to prevent loss of heat, from the mold being heated, to the colder chill plate and to prevent the accumulation of foreign matter which condenses on the chill plate. Another feature is the use of a metal for the shield that will remain solid until the alloy is poured but that will be dissolved by the alloy and mix with it. Preferably the metal of the shield is one of the components of the alloy.

According to the invention a metallic heat shield, preferably in non-flat form and of sheet material is placed on the chill plate within the open end of the mold and resting on the chill plate in only a few locations. The shield substantially covers the area of the open end of the mold and is made of a material that is dissolved by the alloy when it is poured into the mold and that will mix with and be compatible with the alloy. This shield allows for quicker heating of the mold and also prevents collection of impurities on the chill plate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view through a mold and chill plate with the shield therein and all located within a mold heating means.

FIG. 2 is a perspective view of one form of shield.

FIG. 3 is a sectional view similar to FIG. I of a modification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1 the mold 2 is positioned on a chill plate 4 and is surrounded by a susceptor 6 by which the mold may be heated and by which the rate of cooling of the mold may be controlled. The susceptor is surrounded by an induction coil 8 by which the suscepfor is heated and this latter is preferably in the form of a tapped coil such that the top portion of the coil may be energized separately from the bottom portion. The susceptor is preferably in the form of a sleeve with adequate space therein to receive the mold. It will be understood that the entire assembly may be positioned within a vacuum furnace for use in providing the vacuum desirable in casting many of these alloys and in getting the mold to the desired pouring temperature.

The mold consists of a growth portion 10 at the bottom, the bottom end of which is open to the chill plate and rests thereon. The growth portion is connected by a restricted passage defining portion 12 which selects and establishes the growth of a single crystal from the growth portion 10. into the root portion 14 directly above the restricted passage. Above the root portion is the blade portion 16 with a platform portion 18 therebetween. At the top of the blade portion 16 is a shroud 20 and above that is the sprue 22 by which the alloy is poured into the mold. When the entire mold is positioned within the susceptor a cap 24 may be positioned thereon as shown.

Resting on the chill plate and fitting within the bottom open end of the growth portion 10 is a heat shield 26 preferably made of sheet material and preferably in a non-flat condition as shown so that only a few areas of the heat shield are in contact with the chill plate thereby reducing the heat conductivity from the shield to the chill plate. The non-flat condition may be provided by forming a plurality of creases 28 in the shield or alternatively it may be produced by using a corrugated form of shield. The latter substantially covers the area of the chill plate that is exposed to the growth portion of the mold and as such serves to prevent the radiation of heat from the mold directly to the chill plate and also preferably serves to reflect heat back into the mold.

. The material of the heat shield is selected so that it will not melt during the heating of the mold to the desired temperature but will be dissolved by the alloy when it is poured into the mold and comes in contact with the heat shield. Where the device is used in the casting of some of the high-temperature alloys used for example in gas turbine engines, the material used for the shield is preferably one of the materials contained in the alloy. For example, in casting the alloy known as Mar-M-200 which is a nickel-base high-temperature alloy the shield may be either tungsten or columbium since the alloy contains each of these elements. It has been found that the heat shield dissolves readily in the alloy when it is poured.

The heat shield serves not only to minimize radiation from the mold to the chill plate during heating of the mold but also serves to prevent the condensation of gaseous material that could be an impurity for the alloy or could constitute foreign material that would be entrapped within the-alloy during solidification. For example, a material that is caused to evaporate from the mold or the susceptor or any foreign material that may be within the furnace itself may produce this undesired gaseous material that could be deposited on the chill plate except for the presence of this shield. In a particular casting in which the diameter of the growth portion 10 of the mold was 3 inches in diameter a heat shield of columbium formed from sheet stock 0.012 of an inch thick accomplished the expected results. When the alloy was poured, the shield was dissolved completely within the alloy.

The arrangement of FIG. 1 is used for producing single crystal castings. When the alloy is poured and the heat shield is dissolved columnar grains are formed vertically from the chill plate within the growth zone 10. The restrictedpassage above the growth zone selects one of the columnar grains to grow into the root portion 14 and hence into the remainder of the mold. The columnar grains and the dendritic growth within the single crystal are caused to solidify in the desired form by the effect of the chill plate and the maintenance of the proper thermal gradient within the mold during solidification. The actual formation of the single crystal within the mold is described in the above-identified Piearcey patent, U.S. Pat. No. 3,494,709.

The arrangement of FIG. 3 is intended to produce columnar grained cast articles and to this extent the growth portion a connects directly without restriction with the root portion 14a the portions being shown as divided by the dotted line 30 representing the top of the growth zone. Above the platform portion 18a the remainder of the mold is similar to that described in FIG. 1. When a suitable alloy is poured into this mold, the latter having been heated to a temperature above the melting point of the alloy, columnar grains are formed growing vertically from the chill plate and such columnar grains continue directly through the entire mold. Such growth of crystal grains is described in the above-identified ,VerSnyder patent, U.S. Pat. No. 3,260,505.

Although the heat shield and its use are described in the making of shaped articles such as turbine vanes, it has similar utility in casting ingots where the relatively large area of the exposed chill plate makes the heat shield particularly advantageous.

We claim:

1. For casting alloys in which the mold is heated before the alloy is poured, the combination of a shell mold having an open lower end,

a chill plate on which the mold is positioned with the plate closing the open end, and

means for preventing condensation of vaporized impurities including a thin non-flat sheet of metal within said open end positioned essentially horizontallyand substantially covering the portion of the chill plate exposed to the inner cavity of the mold by the open end thereof, said thin sheet having the greater portion thereof spaced above the chill plates to minimize cooling of the sheet and forming a shield for this portion of the chill plate.

2. The combination as in claim 1 in which the thin sheet is formed of a metallic material compatible with the alloy to be poured to mix therewith when the sheet is melted by the alloy poured into the mold.

3. A combination as in claim 1 in which the shield is made of one of the metals in the alloy.

l060ll 0495 

1. For casting alloys in which the mold is heated before the alloy is poured, the combination of a shell mold having an open lower end, a chill plate on which the mold is positioned with the plate closing the open end, and means for preventing condensation of vaporized impurities including a thin non-flat sheet of metal within said open end positioned essentially horizontally and substantially covering the portion of the chill plate exposed to the inner cavity of the mold by the open end thereof, said thin sheet having the greater portion thereof spaced above the chill plates to minimize cooling of the sheet and forming a shield for this portion of the chill plate.
 2. The combination as in claim 1 in which the thin sheet is formed of a metallic material compatible with the alloy to be poured to mix therewith when the sheet is melted by the alloy poured into the mold.
 3. A combination as in claim 1 in which the shield is made of one of the metals in the alloy. 